1. Field of the Invention
The present invention generally relates to light amplifiers suitable for, for instance, optical communications systems and more particularly to a light amplifier which amplifies a signal light introduced into an optical fiber which is doped with an rare-earth ion, such as an erbium ion, which is in an excited state.
2. Description of the Prior Art
Recently, there has been considerable activity in the research and development of a light amplifier using a fiber doped with a rare-earth ion, such as an erbium ion. Hereinafter, such a fiber is referred to as an erbium-doped fiber (EDF). The erbium-doped fiber has a nature such that energy is stored in the excited state when no light signal is applied thereto. The light amplifier using such an erbium-doped fiber has a very high gain in a state in which no light input signal is applied thereto but an exciting light is input thereto. If a light input signal is abruptly applied to the light amplifier, a pulse-like light having a very high peak level may be generated therein. Such a pulse-like light is called a light surge pulse. The light surge pulse occurring in the light amplifier will damage an optical system following the light amplifier.
A conventional method for coping with the above problem uses a monitor which detects a light surge pulse and is provided in the light amplifier. If the monitor detects a light surge pulse, it turns OFF a light source for exciting the erbium-doped fiber provided in the light amplifier.
However, the inverted distribution of the erbium-doped fiber does not directly relate to excitation. Hence, the above conventional method has a restriction regarding the response.